1. Field of the Invention
The subject invention relates generally to castellated locknuts and, more particularly, to an improved castellated locknut structure capable of precise and uniform manufacture and having a thread locking element which is capable of long-term reuse without wear or damage. Hereinafter, castellations shall be referred to alternatively as either castellations or locking members.
2. Description of Related Art
For many years, the fastener manufacturing industry has produced a variety of internally threaded, self-locking locknuts designed to securely engage a threaded shaft. Such self-locking locknuts typically use a thread locking feature which interferes with free rotation of the locknut on the shaft. Means most commonly employed to achieve such interference include locking members which engage the shaft such that, as the locknut is rotated onto the shaft, the locking member is deflected or deformed, permitting forced rotation of the locknut onto the shaft. Resistance within the locking member to the deflection or deformation provides a frictional force between the locknut and the shaft which must be subsequently overcome if the locknut is to further rotate with respect to the shaft.
By "deflection," it is meant that an elastic bending occurs within the locknut such that, upon removal of the locknut from the shaft, the locknut returns to its initial state. In contrast, by "deformation," it is meant that an inelastic bending occurs within the locknut such that, upon removal of the locknut from the shaft, the locknut does not return to its initial state.
Deflection of a locking member is the preferred means for achieving a locking interference, since such a locking member can be reused without any loss in performance.
Four types of locknut structures are well known in the prior art. In the first such structure, commonly called a beam-lock structure, a locknut is provided with a plurality of closely spaced fingers or flexures extending parallel with the axis of the locknut. The flexures are initially deformed along their length toward the axis of the locknut. The flexures include discontinuous threads which engage a shaft. The flexures are deformed or deflected with respect to the body of the locknut as the locknut is inserted onto the shaft.
Beamlock locknuts are frequently limited to having small thread sizes of fine thread pitch. Small variations allowed by thread design specifications in the manufacture of these small-diameter threads limit the required deflection of the flexures necessary to the development of a thread lock. This limited deflection can be accommodated by the beamlock flexures. However, the beamlock structure has disadvantages where large variations are permitted to occur in either or both of the shaft thread and the locknut thread. In particular, large variations require large flexure displacement which, in turn, results in plastic deformation of the flexures and resultant degradation of the thread lock. Also, initial deformation of the flexures with any measure of precision is difficult to accomplish.
A second commonly-known locking structure incorporates a standing collar integral to the top of a locknut through which the thread of the locknut passes. The locknut collar, initially round, is deformed to distort the internal threads, thus creating local interference, friction, and resulting thread lock. The areas of interference in the integral collar are quite small, and, in use, are subject to high stress and resultant wear. Consequently, a locknut constructed according to this method is difficult to produce with precision and tends to degrade rapidly when in use.
A third commonly-known locknut structure incorporates a stricture in the form of a ring or patch of plastic or other nonmetallic material. The stricture functions in the same way as the flexure or deformed threads of the above-described structures by creating an interference with the shaft thread. The nonmetallic locking stricture, however, does not behave elastically, but rather, elastoplastically and, consequently, is prone to wear and degradation when used. Furthermore, the nonmetallic structure can withstand only limited elevated temperatures and, consequently, has limited use in applications requiring high temperatures.
A fourth conventional locknut structure includes a series of widely-spaced castellations, which are integrally formed on the body of the locknut. The castellations are directed parallel to the axis of the locknut, include thread segments, and are bent, subsequent to machining, toward the axis of the locknut. As the locknut is mounted externally to a shaft, the castellations are disposed outwardly as a cantilevered beam. Resistance to displacement produces a thread lock.
The castellated locknut differs from the beamlock locknut described above, in that the castellations are more robust than the flexures of the beam-lock locknut; i.e., the castellations have a greater cross-sectional area. Consequently, the castellations are rigid as compared to the flexures of the beam-lock locknut. Accordingly, the load required to initially bend the castellations toward the axis of the locknut is substantial and, consequently, is difficult to control with precision.
Slots cut into the locknut to produce the castellations interrupt the thread of the locknut and can produce heavily burred threads, particularly at the base of each castellation, which require laborious hand deburring. Furthermore, because the thread is helical, thread segments on the castellations are formed at circumferentially varying distances from the base of the castellations. As a result, adjacent castellations do not have equal cross-sectional base areas. Therefore, adjacent castellations are not deformed equally when subjected to equal loads. This further compromises the performance of the locknut.
Examples of such locknut structures are found in British Patent No. 1,296,887 and U.S. Pat. No. 4,890,965.
Briefly summarizing the disadvantages of the prior art structures, beam-lock locknuts are limited to having small thread sizes, and the deformation of flexures is difficult to control with precision; locking collar structures are subject to wear and degradation, and the process by which the deformation is produced is likewise difficult to control with precision; nonmetallic structures lack durability and are temperature-sensitive; conventional castellated locknut structures are subject to shaft thread damage and wear, lack durability, and are also difficult to produce with precision; and, finally, all include locking features having only a limited capacity to elastically deflect before inelastic deformation occurs, which limits the reusability of the locknut.